Giardia lamblia: Regulation of Cyst Production by Glycosphingolipids
The diplomonad protist, Giardia lamblia, colonizes and replicates in the small intestine of mammals. In humans, Giardia infection (known as giardiasis) can be symptomatic or asymptomatic with the former being associated with fatty stool, abdominal pain, malnutrition, and weight loss. Both cell-mediated and humoral immune responses against Giardia infection are possible, and adaptive responses have been reported to be important for controlling the infection. Non-immune components such as secretory immunoglobulin, intestinal lipids, and bile acids also play significant roles in determining the severity of the infection by Giardia. Giardia exists in two morphologic forms--trophozoites and cysts--and maintains a simple life cycle. Exposure of cysts to gastric acid during passage through the human stomach triggers excystation (morphological changes from cyst to trophozoites), while factors in the small intestine, where trophozoites colonize, induce encystation or cyst formation. The disease is transmitted by water-resistant cysts, which are excreted in the feces of humans and pass on to a new host via contaminated food and water. The production of viable giardial cysts is critical for successful transmission and spreading of the disease. Our laboratory is interested in investigating the lipid metabolism in Giardia because this parasite is unable to synthesize the majority of its own lipids de novo and depends on exogenous lipids for its growth and encystation. In this regard, I am interested in identifying the role of sphingolipids (SLs) during cyst formation. We have shown earlier that, unlike higher eukaryotes Giardia expresses fewer SL genes, which are differentially regulated during encystation. This suggests that there could be a link between SL synthesis and giardial encystation. Furthermore, D-threo-1-phenyl-2-palmitoylamino-3-morpholino-1-propanol (PPMP), an inhibitor of glucosylceramide (GlcCer) synthesis, blocks replication of trophozoites, induces karyokinesis, and inhibits cyst production by Giardia, which indicates that GlcCer plays an essential role in the life cycle of this waterborne pathogen. The goal of my dissertation is to provide understanding of the overall process of cyst production and how the enzyme of GlcCer synthesis (i.e., GlcT1) modulates this process. I have used various molecular and biochemical methodologies to address this question with the help of two separate but interrelated Specific Aims. In Specific Aim 1, I asked if Giardial glucosylceramide transferase enzyme (gGlcT1) regulates encystation and cyst production by Giardia, and the goal of Specific Aim 2 was to investigate whether glucosylceramide transferase regulates ESV (encystation-specific vesicle) biogenesis and cyst viability by maintaining the cellular lipid homeostasis. Briefly, I found that overexpression of gGlcT1 enzyme generated aggregated/enlarged ESVs and congregated cysts with reduced viability. The knocking down of gGlcT1 activity blocked ESV biogenesis completely and produced mostly non-viable cysts. However, when gGlcT1-overexpressed Giardia was knocked down with anti-gGlcT1 morpholino oligonucleotide (i.e., GlcT1 rescued parasites) the enzyme activity, ESV biogenesis, and cyst viability returned to normal, suggesting that the regulated expression of gGlcT1 is important for encystation and viable cyst production. Overexpression of gGlcT1 increased the influx of membrane lipids and fatty acids without altering the fluidity of plasma membranes, which indicates that the expression of gGlcT1 is linked to lipid internalization and maintenance of the overall lipid homeostasis in this unicellular organism. Based on these results, I propose that gGlcT1 is a key player in ESV biogenesis and Giardia cyst viability and that it could therefore be exploited for developing new anti-giardial drugs or a vaccine to control giardiasis.
Mendez, Tavis Lyle, "Giardia lamblia: Regulation of Cyst Production by Glycosphingolipids" (2013). ETD Collection for University of Texas, El Paso. AAI3594353.